Often an engine or motor will operate at speeds different from the required speed of a driven device. Thus, some form of transmission is used to bridge this speed difference (e.g., gears, pulleys etc). Furthermore, the desired speed of the driven device may change substantially while the desired operating speed range of the power supply is narrow (e.g., the internal combustion engine (ICE) in a car operates most efficiently between 1000-3000 rpm, while the driving speeds can vary between −30 mph and 100+ mph). To address such situations the transmission can be designed so that one of several gear ratios is selected at any particular time. In cars these are usually accomplished with manual transmissions (MT) or automatic transmissions (AT). Adding additional gears to a discrete transmission (MT or AT) allows the ICE to operate in a more efficient regime more often, but this is a case of diminishing returns (each additional gear adds less to the total efficiency) and makes the transmission more complicated and expensive.
However, it is still desirable to have the power supply operating at its most efficient operating point regardless of the speed of the driven device. A continuously variable transmission or variator can be used for this purpose. There are many ways to design a variator (e.g., two tapered rollers mounted on parallel rotating shafts pointing in opposite directions and coupled with a belt. Changing the position of the belt alters the speed ratio between the two shafts.) However, there are two shortcomings of variators which prevent them from being widely used in automotive applications; (a) mechanical variators transmit forces via friction and therefore the maximum torque that they can transmit is less than geared transmissions, and (b) variators are typically less efficient than a similarly scaled and well designed geared system.
It is known (U.S. Pat. No. 5,055,094, U.S. Pat. No. 2,955,477, U.S. Pat. No. 3,023,277, U.S. Pat. No. 4,402,237, U.S. Pat. No. 4,864,889, etc) that a variator can be coupled with one or more epicyclic gears to create an infinitely variable transmission (i.e., if the power supply is operating in a constant forward regime, the speed of the powered device can continuously change from forward through zero to reverse.) However, some power (and torque) must always cross the variator. US2011201470 discloses a device that includes a variator that controls the intermediate shafts. However, a need remains for transmissions with large speed ranges and high efficiency.